1. Field of the Invention
The present invention pertains to the hearing aid art, and more particularly, to a hearing aid and method having a feedback microphone in the ear canal and a processor for comparing the spectrum of the actual amplified sound pressure levels at the eardrum to desired levels and controlling amplifiers to achieve the desired levels.
2. Background Art
Most current hearing aids provide one or two stage amplification of ambient sound with the sound level being controlled by the user. Some hearing aids have some frequency response shaping to improve the results for individuals with commonly found types of hearing deficiencies. With the advent of microprocessor controls, more sophisticated hearing aids have been developed that allow the hearing aid to be programmed to the requirements of the hearing impaired individual. The individual is initially tested to determine his hearing characteristics. A preset hearing aid is then selected that provides the closest approximation to the desired results.
Alternatively, a hearing aid is programmed to provide the desired results for the individual patient. A hearing aid having a microprocessor with a programmable read only memory (PROM) may be tested on the patient under actual operating conditions and various sets of amplification instructions tried before one is selected. Once the set of instructions providing the desired results is determined, the PROM is burned in retaining permanently the selected program in the memory. One example of such an application of a microprocessor to a hearing aid is for the control of a bank of band-pass channels that divide the input into the hearing aid into a plurality of frequency bands and then amplify the frequency bands independently according to the program.
A microprocessor controlled hearing aid may be provided with more than one set of operating instructions that are selected either automatically or manually according to ambient operating conditions. For example, the frequency spectra produced during a face to face conversation, a telephone conversation, a conversation in a noisy room, and by music are widely different. A hearing aid set for one operating condition produces less than an optimum output when operating under another operating condition. If the hearing aid has multiple sets of operating instructions and switches automatically, the occurrence of a new listening situation triggers a logic unit transferring the set of instructions for the new situation from the memory to the processor for use in controlling the hearing aid. If the hearing aid has a manual control, the user manually presses a switch that changes the set of instructions utilized by the processor.
Even with the advent of the application of microprocessors to the control of hearing aids to produce better results, all known hearing aids take the ambient sound waves entering the hearing aids and amplify the sound waves in arbitrary ways according to the designs of the hearing aids and then deliver the results to the ear canals of the users. The actual end results in the ear canals are in no way monitored by the hearing aids to change or modify the process to maintain optimum performances. For example peaks in the sound pressure levels at the eardrum may develop due to the output of the hearing aids and the ear canal geometry that cannot be monitored and operated upon by the hearing aids. Such output peaks may exceed the loudness discomfort level of the hearing impaired individual not only at certain frequencies but also in the overall level. This can result in a deterioration in the perception of speech by the individual. The only certain method for not introducing these problems with conventional hearing aids is to set the outputs at less than optimum levels in order to always avoid aberrations that may develop from time to time. The users can, of course, always remove the hearing aids if they are uncomfortable or adjust the controls to less than optimum in order to avoid problems.
Consequently, the need exists for improvements in hearing aids and methods for operating hearing aids that monitor the output of the hearing aids according to actual sound pressure levels at the eardrum and then control the hearing aid performance to prevent exceeding the loudness discomfort level and to optimize performance under the various conditions of input speech loudness, environmental and interfering noise, and other disturbing factors.